1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission installed in a hybrid vehicle.
2. Description of the Related Art
In recent years, in order to reduce fuel consumption for driving an engine and to reduce the amount of exhaust gas that is produced by combusting the fuel, hybrid vehicles have been developed in which an engine and a motor capable of power generation (hereinafter referred to as a motor-generator) are associated with a power transmission mechanism that is connected to driving wheels of a vehicle, driving power during travel of the vehicle is augmented by the motor-generator as necessary, while on the other hand, the power being input to the driving wheels during deceleration of the vehicle is transmitted to the motor-generator so that the motor-generator performs a regenerative operation so as to convert the deceleration energy into regenerated energy which is stored in a battery device as electrical energy.
In some such hybrid vehicles, in order to further improve fuel economy and to further reduce the amount of exhaust gas, a so-called idling stop control is executed in which the engine is automatically stopped by stopping fuel supply to the engine under predetermined conditions such as when the vehicle is stopped.
In the case in which a hydraulically-operated automatic transmission having a torque converter is employed in the above-mentioned hybrid vehicle, hydraulic pressure for a gearing mechanism in the automatic transmission is ensured by an oil pump that is driven by the engine when the engine runs; however, hydraulic pressure for the gearing mechanism cannot be ensured when the above-mentioned idling stop control is executed because the oil pump is also stopped and oil line pressure in the automatic transmission becomes zero.
When the engine is restarted upon receiving an engine starting command under conditions in which the oil line pressure is zero, consequently, the oil pump is also operated, and the oil line pressure steeply increases from zero, which may inhibit unrestricted control operation for oil pressure of a clutch. As a result, the driving torque of the engine may not be smoothly transmitted to an output shaft of the vehicle, which may cause jolting of the vehicle. Moreover, the clutch in the automatic transmission may slip when the oil line pressure steeply increases from zero, which may decrease the life of the clutch.
A hybrid vehicle is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 09-209790 in which the engine is restarted upon detecting release of the brake pedal during an idling stop operation so as to operate the engine-driven oil pump so that the oil line pressure of the automatic transmission is generated before the accelerator pedal is depressed.
Another hybrid vehicle is disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-186585 in which a motor-driven oil pump is employed, instead of an engine-driven oil pump, for generating hydraulic pressure for the gearing mechanism. This document also discloses a control process in which the engine is restarted when the brake pedal and the accelerator pedal are simultaneously depressed during an idling stop operation.
However, details of the control operations for oil line pressure and hydraulic pressure for the clutch associated with the automatic transmission are not disclosed at all in the above-mentioned documents.